And wide tires are not all that good for on-road traction.
If you have ever ridden a 10 speed AND a mountain bike through the slush you will know exactly what I am talking about. The skinny 10 speed tire cuts through the slush and on to the pavement, displacing only enough material to reach the road surface, leaving the rest of your weight for traction. The wider tires do not penetrate as well, and you are sliding around on the surface of the slush.
If a narrow tire and wide tire have the same tread pattern then the narrow one will still resist hydroplaning better.
Where wider tires help is in racecars where you are targeting several Gs and expect to change to new tires regularly despite spreading the loads across the wider width.
Wider tires also help when there are significant chunks of road missing, so that some part of your tire is more likely to be in contact with the road even if half the tire is over a pothole, or in off-road situations where your forward thrust is part newtonian
